More than 3 formatting conditions - select case?

Try this, change the condition ("A","B" etc) to suit

Option Explicit

'START OF CODE


Option Explicit

Private Sub Worksheet_Change(ByVal Target As Range)
    Dim intColour As Integer

    If Not Intersect(Target, Range("A1:A100")) Is Nothing Then
        Select Case Target
            Case "A": intColour = 6
            Case "B": intColour = 3
            Case "C": intColour = 7
            Case "D": intColour = 18
            Case "E": intColour = 15
            Case "F": intColour = 42
            Case Else:    'do nothing
        End Select
        Target.Interior.ColorIndex = intColour
    End If
End Sub

To add this code to your worksheet, do the following:

Copy the code that you want to use
Select the worksheet in which you the code to run
Right click on the sheet tab and choose View Code, to open the Visual Basic Editor.
Where the cursor is flashing, choose Edit | Paste

This code should work for you

Option Explicit


Private Sub Worksheet_Change(ByVal Target As Range)
Dim rng As Range
Dim intColour As Integer

Set rng = ActiveSheet.Range(Cells(2, 8), Cells(Rows.Count, 13).End(xlUp))
    
    If Not Intersect(Target, rng) Is Nothing Then
        Select Case Range("P2").Value
            Case 1: intColour = 6
            Case 2: intColour = 3
            Case 3: intColour = 7
            Case 4: intColour = 18
            Case 5: intColour = 15
            Case 6: intColour = 42
            Case Else:    'do nothing
        End Select
        Target.Interior.ColorIndex = intColour
    End If
End Sub